This invention relates to a vertical cavity, surface emitting laser (VCSEL) with associated output monitoring device and more particularly to such a combination for use in high bandwidth applications.
Lasers, and in particular semiconductor lasers, such as those formed of III-V compounds, are commonly employed as the transmitter in digital communication systems. Lasers of this type are particularly well suited for optical fiber based systems wherein the optical output of the laser is modulated in accordance with an electrical input to the device. The modulated optical signal can be transferred over relatively long distances using current optical fiber transmission systems.
Early lasers used in such applications included edge emitting lasers wherein the cleaved edges of the device provided the reflecting faces of the Fabry-Perrot cavity. Edge emitting lasers, however, have certain limitations with respect to pre-assembly testing and mounting techniques for efficient coupling to small diameter optical fibers.
Surface emitting lasers and in particular vertical cavity surface emitting lasers (VCSELs) have been developed recently and provide an improvement over edge emitting devices for use in optical fiber communication applications. The VCSEL which has its active region located between two reflecting layers, such as Bragg mirrors, emits in a plane normal to one of the two major surface of the laser device. As is known such devices include material of a first conductivity type, for example n-type, for one of the Bragg mirrors and material of a second conductivity type, namely p-type, for the other Bragg mirror. The intermediate active region or layer may include a cladding layer adjacent each of the Bragg mirrors. The Bragg mirrors are typically formed of alternate layers of III-V semiconductor material each layer having a different reflectivity characteristic. Each alternate layer typically has a thickness equal to one quarter wavelength based on the emission wavelength of the active material.
In this structure, and in particular top emitting VCSELs, the back or bottom surface of the device is usually attached to a mounting substrate and the laser output is emitted through the top or front face of the device. An emitting aperture, which may be defined by one of the device contacts, is typically configured to allow alignment with an optical fiber. Such devices are fabricated using well established processing techniques and provide reliable lasers which may be conveniently assembled into optical transmitter units.
The nature of VCSELs, and indeed semiconductor lasers in general, is that the electrical and optical characteristics between each device may vary slightly. The optical output taken as a function of input current during lasing action represents a steep slope and minor variations in the operating environment can result in significant output changes. For this reason, it is common to include a monitoring diode or monitor chip with a VCSEL, wherein the monitor chip is arranged to receive a representative portion of the optical output. This representative output can be used to calibrate each laser device or it can be used in a feedback mode to control the optical output of the laser. Control may be required to ensure that the optical output of the laser falls within a preset limit such as might be required by xe2x80x9ceye safexe2x80x9d regulations prescribed by Standards Agencies. Typically the monitoring diode will be a phototransistor such as a PIN device having a sensitivity curve generally matched to the wavelength output of the laser.
Laser/monitor combinations are frequently mounted in a specially designed package such as a T0-46 can which has a mounting base with insulated connector leads and a sealed cover. The cover has a window of glass or other suitable transparent material over a central portion of the top such that the window is aligned with the emitting aperture of the lasing device. One such combination is described in U.S. Pat. No. 5,812,582 which issued Sep. 22, 1998 to Gilliland et al. In the U.S. Pat. No. 5,812,582 patent the photodiode is mounted on an insulated substrate that is positioned within in a T0-46 can, or the like. A large portion of the top surface of the photodiode is covered with a metal layer or mask. A VCSEL is electrically attached to the mask by solder or conductive epoxy and one of the contacts to the VCSEL, i.e. the back contact, is made by way of the mask. The top or emitting surface contact is through a wire bond to one of the isolated connectors in the T0-46 can.
The bandwidth capacity of current optical fibers far exceeds the bandwidth utilized by present day communication systems. Accordingly, there is a continuing effort to increase the data rate of communication systems in order to make better usage of optical fiber capabilities. Since the laser transmitter represents an important aspect of the complete communication system it is important that the switching rate of the laser be as high as possible. One factor which effects the switching rate in high speed devices is the parasitic capacitance of the VCSEL, the monitoring chip and the mounting configuration.
A further important consideration, of course is the cost of the optical transmitter or laser/monitor assembly. This cost includes the material processing costs as well as the cost of assembling the devices and in accurately aligning the device in relation to an optical fiber.
It is, accordingly, an object of the present invention to provide a low cost VCSEL/monitor device with reduced parasitic capacitance for high bandwidth applications.
In a preferred embodiment of the present invention there is provided a VCSEL/monitor assembly in which the VCSEL has both p-type and n-type contacts on the top or emitting face and is mounted on a monitoring diode utilizing little or no metallisation in the mounting process.
Therefore, in accordance with a first aspect of the present invention there is provided a vertical cavity surface emitting laser (VCSEL) and photo detecting monitor assembly comprising: a photo detecting monitor chip having a first photo detecting face and a second face parallel thereto; a top emitting VCSEL, mounted on the first face of the monitor chip, the VCSEL having both p-type and n-type contacts on a top surface thereof, and means associated with the assembly to direct a portion of the VCSEL emission to the first face of the monitor chip.
In accordance with a second aspect of the present invention there is provided a method of assembling a top emitting vertical cavity surface emitting laser (VCSEL) and photo detecting optical output monitoring chip pair comprising: providing a monitoring chip having a photo detecting surface; attaching a top emitting VCSEL to the detecting surface, the VCSEL having p-type and n-type contacts on the top surface; and providing means to contact the monitoring chip and the VCSEL.